question - spoiler
Regarding how the boy died - wouldn't he have had head injuries and possibly blood on the wall from running into it so hard that might have prompted further investigation/consideration than just "she broke his neck"? Granted there was a witness (unreliable and with a nasty axe to grind though she was) but surely the coroner/police have a duty in such cases to look into things - the fact that he was known for this behaviour might have come out.
Some have wondered how Tilly got to go overseas - I presumed her father sent her after he sent her to school, to get rid of her.